Concentration, Life Stage, Feeding, Density, Flow, and Strain Effects on Formalin Sensitivity in Rainbow Trout (Oncorhynchus mykiss)

SIMPLE SUMMARY: Chemical compounds such as formalin are commonly used to prevent fungal infections on fish eggs and treat external parasites on hatchery-reared fish. However, exposure to formalin can cause mortality, depending on the concentration used and the rearing conditions under which the fish are being treated. Strains within a species can also differ in their sensitivity to formalin. Four experiments were conducted to evaluate egg and fingerling mortality of four strains of rainbow trout (Oncorhynchus mykiss) exposed to a range of formalin concentrations. How mortality was affected by the conditions under which rainbow trout were being treated was also examined. Strains exhibited differential sensitivity to formalin in both the egg and fingerling life stages. Hatchery rearing conditions were also found to differentially affect mortality, both within and across strains. Therefore, formalin concentration, rearing conditions, and potential strain differences in sensitivity should be considered prior to initiating large-scale formalin treatments. ABSTRACT: Formalin is one of the most widely used and effective chemotherapeutic compounds for treatment of fungal infections and external parasites of fish eggs and fish. However, exposure to formalin can cause mortality in eggs and fingerlings, dependent upon the concentration used and the rearing conditions in which fish are treated. Additionally, strains within a species can exhibit differential susceptibility to formalin. Four experiments were conducted to evaluate the differential sensitivity to formalin of four rainbow trout (Oncorhynchus mykiss) strains in both the egg and fingerling life stages. Eggs were exposed to concentrations of 1667, 2000, and 5000 ppm formalin, and sensitivity differed among the strains when formalin concentration exceeded 2000 ppm. Exposure to higher formalin concentrations (i.e., 5000 ppm) as eggs did not increase mortality when fish were re-exposed to concentrations of 0, 167, 250, or 500 ppm formalin at 77 mm total length (TL). Fish size affected formalin sensitivity, with larger fish (128 mm TL) exhibiting higher rates of mortality than fish ≤ 77 mm TL when exposed to 250 ppm formalin. The effects of crowding, feeding, flow, and density on the formalin sensitivity of 77 mm TL fish were also investigated. Mortality increased in fish crowded away from the inflow to prevent contact with formalin as it entered the tank, potentially the result of an increase in density index within the crowded tanks. Feeding fish on the day they were treated caused mortality to increase by 5.4 to 8.8% in fish exposed to 167 and 250 ppm formalin, respectively, and mortality differed by strain. Reducing flows by half resulted in doubled to quadrupled mortality, and increased densities resulted in increased mortality in some strains but not others. Hatchery managers should consider what effect rearing conditions and formalin concentrations might have on the strain of fish being treated, prior to large-scale treatment.